Polymerization of aspartic acid and aspartic-acid precursors, such as maleic acid plus ammonia, to produce first polysuccinimide, then polyaspartate by mild alkaline hydrolysis of the imide rings, has been the subject of commercial research and development for more than two decades. Much of this effort is summarized in U.S. Pat. Nos. 5,981,691 and 6,495,658 to Sikes and coworkers (1999, 2002).
These polyanionic polymers have the advantages of ready biodegradability and good biocompatibility. Although research and development of polysuccinimide and polyaspartate on a large scale has occurred in numerous companies over this interval, successful commercialization of the molecules has been limited by technical difficulties of several kinds.
Bayer Company has used the maleic-plus-ammonia route to produce molecules of low molecular weight (approximately 2000 to 3000 Da). In addition, these molecules are branched rather than linear in morphology, which tends to hinder environmental degradability. These molecules have been introduced into a number of products, including detergents, in which the polyaspartates provide dispersancy and protection against redeposition of mineral deposits.
The maleic-plus-ammonia route, however, is not extendable beyond the range of low molecular weights. This problem, plus the branched morphology of the polymer products, tends to limit the utility and performance of these molecules in many markets.
Other companies, for example Rohm and Haas, Solutia, and Donlar Corporation, have focused on polymerization of aspartic acid itself. The dry thermal polymerization of aspartic acid results first in polysuccinimides, then polyaspartates following ring-opening via mild alkaline treatment, that are somewhat larger in size (molecular weights around 3,000 to 5,000), and also less branched, than those described above. Donlar introduced this type of polyaspartate in some detergent markets and also in an oilfield application, and has made an effort to introduce the polyaspartate into agricultural markets as a soil additive and growth enhancer.
Mukouyama, in U.S. Pat. No. 6,380,350 (2002), teaches the polymerization of aspartic acid in water via heat and pressure in an autoclave. The product is polyaspartic acid, obtained directly, without production of the intermediate polysucciniinide. Reasonably high yields of low Mw (2 to 6.5 kDa) polyaspartic acids were reported.
Many if not most markets often require larger molecules, in numerous cases much larger molecules, ranging from 10,000 to 100,000 or more in molecular weight. The principal approach to this issue has been the use of acid catalysis, typically phosphoric acid or polyphosphoric acid, at up to 30% or higher by weight of the aspartic acid monomer, as an agent that promotes rapid polymerization. In this approach, the polymers attain a larger size before chain-lengthening is terminated. Such termination is generally due to thermal decomposition of the amino termini, which are entirely absent in thermally produced polyaspartates upon completion of chain lengthening.
Molecules in the range of 30,000 Da and somewhat higher are achievable via acid catalysis. An added benefit of this approach is that color formation tends to be suppressed under these conditions, resulting in polymers of favorable, light tan to off-white color. However, the use and subsequent removal of the acid catalyst adds significantly to cost.
Attempts to produce copolymers of aspartate and succinimide by substoichiometric, mild-alkaline ring-opening of the imide residues of polysuccinimide were unsuccessful (Wolk et al., 1994). Treatment of an aqueous slurry of polysuccinimide particles residues with NaOH, for example, produced a soluble phase containing fully ring-opened polyaspartate and an insoluble phase of succinimide polymer. The alkaline attack appeared to bring the surficial polysuccinimide molecules into solution, where they rapidly became fully ring-opened, leaving a residual particle of insoluble polysuccinimide. There was essentially no evidence of copolymer produced via this approach.
Use of ammonium hydroxide for ring-opening of polysuccinimide was reported by Koskan and Meah in U.S. Pat. No. 5,219,952; however, only polyaspartate was described as the product. When a large excess of liquid ammonia under pressure was used for ring-opening of polysuccinimide, a homopolymer of asparagine was produced (Ma, 2002; U.S. Pat. No. 6,365,706).
Copolymers of aspartic acid and succinimide containing undefined levels of asparagine residues have been described as reaction products of the maleic-plus-ammonia route to low Mw, branched polysuccinimides, resulting from the use of large excesses of ammonia. In addition, when the temperatures of polymerization were too low or reaction conditions were otherwise insufficient (e.g., too short an interval of heating) to completely effect the ring-closure of succinimide residues, aspartic acid residues were reported to occur in the product copolymers. (See e.g. Groth et al., U.S. Pat. Nos. 5,493,004, 5,594,077, 5,714,558, and 6,054,553; Kroner et al., U.S. Pat. Nos. 5,548,036 and 5,639,832.) These reports describe copolymers produced as undesired and undefined side products, rather than the defined aspartate-asparagine-succinimide copolymers disclosed herein.
Derivatization
Another useful feature of polysuccinimides is the reactivity of the imide rings to derivatization. Nucleophiles, such as amino compounds, readily form covalent linkages to the polymer backbone via amide bond formation at the carbonyl carbon, by attacking the imide linkage to the imide nitrogen. However, due to the low water solubility and wettability of these compounds, most efforts to produce derivatives of polyaspartate via this route (i.e. derivatization of polysuccinimide, followed by alkaline ring-opening of unreacted succinimide residues) have been conducted in organic solvents such as dimethyl formamide and dichloromethane, in which the polysuccinimide and usually the nucleophilic additive are both soluble. Use of such solvents is costly and also militates against use of the products in many markets, for example personal-care and biomedical markets, in which even traces of organic solvents are not allowable. For these reasons, as well as the reasons already cited regarding the homopolymers themselves, derivatives of the polysuccinimides and polyaspartates have not found marketable applications to date.
Attempts have been made to functionalize polysuccinimide in water via nucleophilic addition of amino compounds to an aqueous slurry of polysuccinimide. As the nucleophile adds to the polyimide, the latter is gradually solubilized, and can then be further functionalized much more readily in water. The problems with this approach include production of heterogeneous molecules (surficial polysuccinimides of the polysuccinimide particles tend to become over-derivatized, the others under-derivatized), plus the overall slowness and inefficiency of the process. Consequently, most of these approaches have not been pursued.
First-Generation, Water-Soluble Copolyimides of Amino Acids
Copolymers of aspartate and succinimide were disclosed by Sikes and coworkers (1999, U.S. Pat. No. 5,981,691; U.S. Pat. No. 6,495,658; both incorporated herein by reference). In these approaches, copolymers were produced via thermal copolymerization of aspartic acid and sodium aspartate, leading directly to imide-containing copolymers, and obviating the intermediate production of polysuccinimide. The copolymers are highly water-soluble and thus readily derivatized via nucleophilic addition in water, enabling economic production of high-performance derivatives having favorable environmental profiles.
However, several inherent problems remained. For example, synthesis of the copolymers by this method results in significantly branched, low-molecular-weight, moderately-to-highly colored (light tan to dark reddish) products. The only methods disclosed for achieving somewhat higher Mw were inclusion of crosslinking and chain-extending comonomers, such as lysine, and inclusion of a preformed polyaspartate in the polymerizing mixture of comonomeric aspartic acid and sodium aspartate.
In addition, the disclosed synthetic processes require the pH and ionic content of the reactant solutions, prior to thermal polycondensation, to be controlled within narrow limits. This restriction prevents utilization of strategies such as acid catalysis to promote production of higher molecular weight forms of polysuccinimide and polyaspartate. Acid catalysis also provides the advantage of producing polysuccinimides of light color (light tan to cream-colored), as mentioned above.
Accordingly, currently available methods of producing water soluble aspartate-succinimide copolymers enable the production of only low Mw, branched forms of the copolymers.